This achievement is story-related and can't be missed.
After getting PAT into your party, go to the Great Wave headquarters, where the fight arena is located. Upon arriving there you'll fight some enemies with the help of local guards.
When you've dispatched these enemies, go to the top-left door, which will take you to the very place where it all started: the corridor leading to Ive's bedroom. Take the door to your right.
Keep going right, fight some enemies on the students hall, keep right and you'll eventually find Aderyn and Ichabo. The latter will flee and you must fight Aderyn. Upon defeating her, achievement should unlock after conversation is over.